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Family loyalty driving sperm teamwork?

Jan. 23, 2007
Courtesy Unviersity of Sheffield
and World Science staff

A sper­m’s quest to fer­ti­lize an egg is a lone­ly one, as sci­en­tists tra­di­tion­ally por­tray it. But for rats and mice, that’s not al­ways true: the sperm can join in­to teams, de­ter­mined by who they came from, re­search­ers have found.

Left: Ar­rows mark two sperm groups in a lab dish from the house mouse, Mus mus­cu­lus. Right: a sche­mat­ic di­a­gram show­ing how they are found to hook to­geth­er. (Cour­te­sy S. Imm­ler et al. / PLoS One) 
The find­ings are a new ad­di­tion to grow­ing ev­i­dence that sperm can co­op­er­ate, the sci­en­tists said.

Rat and mice are pro­mis­cu­ous an­i­mals. Con­se­quent­ly, more than one male’s sperm may be in the fe­male at a giv­en time, fight­ing its way to­ward the egg in an at­tempt to be the one that can fer­ti­lize it.

Thus, the com­pe­ti­tion is on two lev­els: sperm race against each oth­er as in­di­vid­u­als, but sperm from one male al­so com­pete against those from anoth­er. The sperm ex­hib­it a sort of fam­i­ly loy­al­ty, the re­search­ers ar­gued: they help their own kin, so that even if one does­n’t tri­umph, it helps en­sure that anoth­er one from the same male does. 

For sper­m—the male sex cells that are in­sem­i­nat­ed by the mil­lion­s—co­op­er­a­tion makes more sense when fe­ma­les are pro­mis­cu­ous, ac­cord­ing to Si­mone Imm­ler and col­leagues at the Uni­ver­si­ty of Shef­field, U.K., au­thors of the stu­dy.

Rat and mouse sperm have a high­ly spe­cial­ised de­sign that makes this pos­si­ble, Imm­ler’s group ex­plained. Like some oth­er pro­mis­cu­ous spe­cies, they have a particularly well-de­vel­oped hook-shaped sperm head that helps sperm link in­to groups of five to 100. The groups swim faster and stronger than in­di­vid­u­als.

“When the pres­sure from ri­val ma­les is high, in­di­vid­u­al sperm will co­op­er­ate with one anoth­er to en­sure that at least one of their sib­lings suc­cess­ful­ly reaches the fe­male egg,” said Imm­ler, whose team al­so not­ed that sperm co­op­er­a­tion has been found in opos­sum and in­sects called fish­flies.

The findings ap­peared Jan. 23 in the on­line re­search jour­nal Pub­lic Li­brary of Sci­ence One.

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A sperm’s quest to fertilize an egg is a lonely one, as scientists traditionally portray it. But for rats and mice, that’s not always true: the sperm can join into teams, determined by who they came from, researchers have found. The findings are a new addition to growing evidence that sperm can cooperate, the scientists said. Rat and mice are promiscuous animals. Consequently, more than one male’s sperm may be in the female at a given time, fighting its way toward the egg in an attempt to be the one that can fertilize it. Thus, the competition is on two levels: sperm race against each other as individuals, but sperm from one male also compete against those from another. The sperm exhibit a sort of family loyalty, the researchers argued: they help their own kin, so that even if one doesn’t triumph, it helps ensure that another one from the same male does. For sperm—the male sex cells that are inseminated by the millions—cooperation makes more sense when females are promiscuous, according to Simone Immler and colleagues at the University of Sheffield, U.K., authors of the study. Rat and mouse sperm have a highly specialised design that makes this possible, Immler’s group explained. Like some other promiscuous species, they have a particularly well-developed ‘hook shaped’ sperm head that helps sperm link into groups of five to 100. The groups swim faster and stronger than individuals. “When the pressure from rival males is high, individual sperm will cooperate with one another to ensure that at least one of their siblings successfully reaches the female egg,” said Immler, whose team also noted that sperm cooperation has been found in opossum and insects called fishflies. The results appeared Jan. 24 in the online research journal Public Library of Science One.